Experimental data produced by this grant has confirmed the vital role of sodium as related to vascular tone. The correlation of sodium levels with photographic studies of the microcirculation, cardiac output, and peripheral vascular resistance suggests the following: (1) A distinct increase in the diameters of both arterioles and venules in the hyponatremic state (average 10-12 percent). (2) An increase in cardiac output with a decrease in peripheral vascular resistance is consistently observed in the hyponatremic state. (3) Blood pressure levels consistently drop as the level of sodium in the serum and vascular wall is reduced. (4) The clinical implications of these findings are important in a wide variety of cardiovascular disease.